The invention relates to a hydrophobic silica, to a process for its preparation and to its use in silicone rubber.
It is known to use pyrogenic silica that has been rendered hydrophobic as a filler in silicone rubber (DE 199 43 666 A1).
U.S. Pat. No. 6,331,588 describes LSR-silicone rubbers which contain pyrogenic silicas as filler. In order to avoid the undesirable influence of the silanol groups on the rheological properties of the silicone rubber it is necessary according to U.S. Pat. No. 6,331,588 to render the surface of the pyrogenic silica hydrophobic.
According to the prior art, in the case of LSR (liquid silicone rubber), either a hydrophilic silica is rendered hydrophobic in situ and at the same time exposed to very high shear forces so that the viscosity and the flow limit can be lowered, or a silica that has already been rendered hydrophobic is exposed to high shear forces for the same reason.
A hydrophilic silicon dioxide (silica) doped with potassium by means of aerosol is known from DE 19650500 A1.
A surface-modified silicon dioxide doped with potassium is generally known from EP 1236773 A2. The KCl solution used therein has a concentration of 0.5 KCl.
The K content of the hydrophilic silicon dioxide is 300 μg/g. A specially designed surface-modified, potassium-doped silicon dioxide is not described in EP 1236773 A2.